Gaseous discharge lamps have long been operated from power lines through transformers, auto-transformers and passive components such as capacitors and chokes. Such devices are often quite bulky, heavy and have limited regulation capability. They are, of course a-c devices and are not suitable for d-c use.
Electronic ballasts having semiconductors, magnetic components and passive components may also be used to supply the lamp with the voltages and currents which it requires to start, warm-up and operate. Magnetic and reactive components may be operated at high frequencies (above 20 kHz, for example), thus reducing their size and weight. The input power may be from a low voltage d-c source. Prior electronic ballasts are described in Engel et al U.S. Pat. No. 3,590,316 which incorporates a solid state wattmeter circuit; in Dendy et al U.S. Pat. No. 3,999,100 which uses a switching regulator and commutator; and in Herzog U.S. Pat. No. 3,969,652 which includes a switching regulator, inverter and starting circuit. These ballasts have the following drawbacks:
1. Poor starting with some types of lamps because of limits in the voltage which the ballast can safely withstand and because of a lack of a "follow-on" energy source. This is needed to aid in sustaining the arc between the time that the starting pulse is applied and the time that the ballast establishes a steady-state arc current. PA1 2. Limited hot restrike capability. When a lamp is fully warmed up, its internal pressure will be many times greater than when the lamp is cold. The magnitude of the high voltage pulse required to start the lamp when it is hot is dependent upon this gas pressure. For example, some lamps require 15 kv to cold start and 35 kv to hot start. Previous methods of starting have been limited in the highest starting voltage they could produce, and hence could start, but not hot-restrike a lamp. In many applications, it is desirable to have the lamp restrike immediately after a power interruption, without a waiting period for the lamp to cool down. PA1 3. Certain of the previous methods use switching regulators cascaded with inverter circuits. The input voltage suffers two or three junction drops and efficiency is impaired, especially for low voltage sources. PA1 4. The prior circuits lack protection of lamp or ballast from high d-c transients developed in the lamp circuit just after ignition. These transients are due to formation of a cathode spot on only one lamp electrode. This causes the lamp to act as a rectifier just after starting for periods of about one second. The resulting flow of d-c current in the lamp and ballast output circuit can saturate ballast magnetic components and damage the lamp and/or ballast if high currents flow. PA1 5. The control of lamp wattage has often required the sensing of lamp voltage directly, which can result in exposure of low level circuits to the high starting voltages. Other approaches sense currents at intermediate points within the inverter circuitry where transients are often present and degrade the accuracy of measurement. Some schemes use costly analog multipliers or logarithmic techniques to determine the product of voltage and current, and hence, wattage. PA1 6. Certain types of ballasts provide sine-wave (or non-squarewave) outputs of various frequencies. These can create acoustic arc resonance effects in some lamps, causing unstable arcs and possible destruction of the arc tube. See, for example, J. H. Campbell, "Initial Characteristics of High-Intensity Discharge Lamps on High-Frequency Power", Illuminating Engineering, December 1969, p. 713 and C. F. Scholz, "Characteristics of Acoustical Resonance in Discharge Lamps", Illuminating Engineering, December 1970, p. 713. Current square-waves provide higher lamp and ballast efficiency. PA1 1. To provide an electronic ballast capable of starting, operating and restriking gaseous discharge lamps from a source of d-c power such as low voltage batteries. PA1 2. To provide an electronic ballast of small size and weight compared to powerline low frequency magnetic ballasts so as to be well-suited for portable and vehicular applications. PA1 3. To provide substantially a square-wave current drive to the lamp to minimize flicker and acoustic arc resonance, and to maximize efficiency of the lamp and ballast power components. PA1 4. To regulate lamp current during lamp warm-up and lamp wattage during lamp operation. PA1 5. To derive lamp wattage control without use of costly multiplier circuitry. PA1 6. To provide means for starting and hot restriking of the lamp without application of high voltage pulses directly to the lamp's main electrodes. PA1 7. To provide means to protect ballast semiconductors from high voltages present during starting. PA1 8. To prevent the flow of d-c current in the lamp circuit during the early part of the warm-up period. PA1 9. To provide means for operating various types of lamps from a single ballast via a switching device. PA1 10. To provide means to turn the lamp and ballast on and off in response to an externally applied low level control signal. PA1 11. To provide high efficiency power conversion for low voltage sources.